1. Field of the Invention
The present invention relates generally to systems and methods for paper moisture and basis weight measurement, and more particularly for continuous, wide coverage measurement of paper moisture, and basis weight using a camera assembly.
2. Discussion of Background Art
Paper manufacture is a very energy and waste-intensive industry. A typical paper machine makes paper that is three-hundred inches wide at speeds of up to five-thousand feet per minute. In order to produce a high quality paper product manufacturers would prefer to monitor as many of the properties of a paper web as possible during manufacture to ensure uniformity and high quality throughout the entire web.
During manufacture there are about a dozen properties that are preferably measured, including moisture, thickness, weight, formation, fiber orientation, color, and printability. In an attempt to meet this need, some current on-line paper web measurement systems use a slow-moving scanning platform containing a suite of sensors for measuring many of these properties, including moisture. Moisture is a critical parameter for paper. For instance, since paper is very hydroscopic and too much moisture affects printability, manufactures tend to take a conservative approach and over-dry their paper products, greatly increasing energy costs.
Methods for making moisture measurements based on near-infrared absorption techniques where originally developed in the sixties, e.g., U.S. Pat. No. 3,405,268. Other patents further refined these method by using specific wavelength combinations and mathematical formulations, e.g. U.S. Pat. Nos. 3,551,678, 3,793,524, 3,851,175, 4,052,615, and 4,823,008.
Current practices for online paper measurement of moisture and cellulose content (which is proportional to basis weight) via absorption techniques, incorporate several lead sulfide sensors for measuring wavelengths between 1.8 xcexcm and to 2.2 xcexcm. These sensors are mounted on a scanning platform in close proximity to the paper web, and take point measurements. Point measurements are those which only look at one xe2x80x9cpointxe2x80x9d of about an inch square at any one time. This small sensing area, along with the fact that it takes about a minute or more for the scanning platform to move across the width of a typical moving paper web, can result in the production of thousands of feet of paper before a point sensor can cross the full width of the web even once. In such point measurement based systems, typically less than two percent of a total paper web area is actually measured.
In another approach, Charged Couple Device (CCD) linescan camera technology has been used on paper webs for inspection of visible defect flaws and non-uniformity detectable in a visible range, e.g. U.S. Pat. Nos. 4,950,911 and 5,563,809. However, CCD sensor technology, which is based on Silicon, is not capable of detecting those wavelengths in the near-infrared range needed for accurately measuring paper moisture or cellulose content.
If manufacturers had a method for conducting measurements for moisture and cellulose content over one hundred percent of a paper web, paper-drying time could be substantially reduced, resulting in significant energy savings, and improved paper quality.
In response to the concerns discussed above, what is needed is a system and method for 100% paper moisture and cellulose measurement that overcomes the problems of the prior art.
The present invention is a system and method for 100% measurement of moisture and basis weight on moving paper. Within the system of the present invention, a camera assembly monitors a water absorption wavelength radiated by a substance, such as a paper web, through a first array of photo detectors. The camera assembly monitors a cellulose absorption wavelength radiated by the substance through a second array of photo detectors. The camera assembly monitors a reference wavelength radiated by the substance through a third array of photo detectors. A single optical axis system is used to image the web from a distance and provide the same field of view to each array in the camera assembly. Optical filters, which could include dichroic mirrors or a prism, are used to filter the specific wavelengths to be measured with each of the three arrays. The filters could also be coated directly on the array photo detectors. A computational device or lookup table is used to calculate the moisture content and basis weight of the substance from the water absorption, cellulose absorption, and reference wavelength data received from the camera assembly. Supporting electronics interface signals from the three arrays with the computational device or lookup table. In other aspects of the invention, the first array has a band-pass filter in its optical path transparent to a water absorption wavelength such as 1.45 xcexcm or 1.94 xcexcm wavelength; the second array has a band-pass filter in its optical path transparent to a cellulose absorption wavelength such as 1.57 xcexcm or 2.1 xcexcm; the third array has a band-pass filter in its optical path transparent to other wavelengths in the near-infrared range which do not include water or cellulose absorption wavelengths, such as 1.3 xcexcm, or 1.8 xcexcm.
In other aspects of the invention the first, second, and third linear arrays consist of Indium Gallium Arsenide (InGaAs) linear arrays. These arrays consist of a plurality of photodiodes, sensitive to the near-infrared spectrum. Unlike CCD arrays, InGaAs arrays are sensitive to wavelength between 0.9 xcexcm and 2.2 xcexcm. Therefore, these arrays are very suitable for measuring all the wavelength of interest for this application. Furthermore, the array nature of this sensor technology makes it practical to build a high resolution linescan camera system with the use of optics and supporting electronics for 100% inspection of high-speed web processes such as paper.
In other aspects of the invention a light source, which radiates energy within at least an infrared spectral band, is used to illuminate a paper web. The current invention can be used in a transmission mode where the light source is on one side of the web and the camera assembly is on an opposite side, or in a reflection mode where the light source is on a same side of the web as the camera assembly.
In other aspects of the invention, depending on a width of the web, the size of the arrays, a desired resolution of measurement, a distance between the camera assembly and the web, and a field of view of the camera assembly, several camera assemblies might be needed to monitor a full width of the paper web.
The system and method of the present invention are particularly advantageous over the prior art because they permit a full width paper web to be continuously monitored for moisture and basis weight during manufacture. Such monitoring enables faster detection of moisture and basis weight irregularities so that manufacturers can correct problems before miles of off-quality paper are produced. Such monitoring also helps reduce energy costs and waste because the paper web would not be over-dried as now often happens. Integrating the present invention with existing paper web visible defect inspection systems is straightforward, since the same light source and camera enclosures can be shared between the two systems. It might also be practical in some cases to integrate the present invention with other array sensors such as CCD arrays through a single optical axis to measure other properties of the web in parallel with those described here.
These and other aspects of the invention will be recognized by those skilled in the art upon review of the detailed description, drawings, and claims set forth below.